Nitrogen metabolism and transport in the Arbuscular mycorrhizal interaction

The Arbuscular Mycorrhiza is arguably the world's most important interaction. The roots of over 70 % of all known land plant species form this mutualistic interaction with fungi of the phylum Glomeromycota. Arbuscular mycorrhizal (AM) fungi can act as biofertilizers, bioprotectors and bioregulators of plants and have gained increasing attention for their potential role in sustainable agriculture and in the restoration and bioremediation of contaminated and disturbed sites. In this interaction plants take up mineral nutrients from the soil through their associated AM fungi and transfer in exchange for their beneificial effect on nutrient uptake photosynthetically fixed carbon to the fungus. Nitrogen (N) is known to be transferred from the fungus to the plant in the AM interaction, yet its metabolism, storage and transport in the symbiosis are poorly understood. Here, we report new findings about the N metabolism and transport in the AM symbiosis by analyzing fungal gene expression with quantitative polymerase chain reaction (Q-PCR). In vitro mycorrhizas of Glomus intraradices and Ri T-DNA-transformed carrot roots were grown in two-compartment Petri dishes. Different experiments were carried out to measure the effect of different carbon (C) or nitrogen (N) sources on fungal gene expression. The RNA was extracted from the ERM and processed for Q-PCR using gene specific primers. Inorganic nitrogen is taken up by the fungus with its extraradical mycelium (ERM), is incorporated into amino acids, and translocated from the ERM to the intraradical mycelium (IRM) as arginine, where it is broken down to an inorganic form via the catabolic arm of the urea cycle and transferred to the plant without C (Govindarajulu et al., 2005; Jin et al., 2005). Consistent with the proposed mechanism, the genes involved primarily in nitrogen assimilation were highly expressed in the ERM whereas the mRNA transcripts levels of genes associated with the breakdown of arginine were low in the ERM. The expression levels of these genes are regulated by the C availability for the mycorrhizal fungus and by an exogenous supply of N to the ERM.M.S.Includes bibliographical references (p. 34-38)by Sulbha Choudhar